Charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) devices, and infrared imagers, which may be referred to generally as Solid State Area Array Imaging Devices (SSAAIDs), are used to capture images received in the form of light. They are currently widely used for both defense and commercial purposes. Some popular uses include digital cameras, scanners, cell phones, and surveillance devices.
SSAAIDs contain pixels arranged in a grid, which is referred to as a Focal Plane Array (FPA). Each pixel of an SSAAID generates and holds an amount of charge proportionate to the intensity of light incident thereon and the length of time that light was allowed to fall on the pixel using an integration circuit.
An integration circuit performs the mathematical operation of integration with respect to time. Said another way, the output voltage of an integration circuit is proportional to the input voltage, integrated over time (Output∝∫Input). In the case of a pixel, the input voltage is generated by the impact of photons on a detector. The charge handling capacity of such a circuit is determined by voltage, integration time, and capacitance of its capacitor(s).
Current SSAAIDs are limited in their ability to provide acceptable images in moderate to low light level conditions as well as in high light level conditions by the dynamic range of the integration circuit of the pixels. In low light level conditions, where there are relatively few incoming photons incident on any given pixel, the signal-to-noise ratio (SNR) of the output is very low, resulting in a grainy/noisy image in dark areas of the image. Moreover, in low SNR situations, other variables can also create non-uniformities in the images where the signal levels are not sufficient to overcome the sensitivity anomalies.
Pixel integration circuits may also become saturated in high light level conditions. When a large amount of light hits a pixel, the integration circuit of that pixel, and even those of nearby pixels due to a phenomenon referred to as “blooming”, become saturated, a situation that results in the integration circuit ceasing to be able to capture additional information. Saturation results in washed out images or portions thereof. Although anti-blooming circuits may be used to help reduce the impact of one or a cluster of saturated pixels on others, to increase high light level performance of a given pixel requires increasing the capacity, or well size, of its integration circuit, thereby preventing saturation over a given interval of time.
Prior art FPAs have used shorter integration times to provide better low gain, or high light, performance, but are less sensitive as a result and therefore less able to capture low light level conditions.
Said simply, existing integration circuits used in pixels have limited dynamic range, resulting in a loss of details in high and low light level areas of an image, and are more complex than necessary. What is needed, therefore, is an integration circuit that is simpler while offering more dynamic range, without loss of sensitivity, than currently available and a method of operating the same.